The present invention relates to a refractive index distribution type light transmitting device having both high transparency and heat resistance which used to be difficult to realize with conventional optical resin materials.
Heretofore, as a refractive index distribution type plastic light transmitting device, one having a distribution formed by mixing and polymerizing a monomer capable of presenting a non-crystalline polymer and a non-polymerizable material having a different refractive index, has been known as disclosed in WO93-08488. Further, JP-A-8-5848 discloses that using a non-crystalline fluorine-containing polymer having a fluorine-containing alicyclic structure in its main chain and containing no C-H bond, as a matrix resin, a diffusion material which is melt-diffusible in this resin and which has a refractive index different from this resin, is distributed to obtain a refractive index distribution type plastic light transmitting device.
There is a phenomenon such that when a diffusion material having a different refractive index is melt-dispersed to form a refractive index distribution, the glass transition temperature (Tg) of the optical resin lowers, and the heat resistance decreases. Especially with a plastic light-transmitting device having a diffusion material with a not so high refractive index dispersed therein, it is necessary to increase the content of the diffusion material in order to increase the numerical aperture (NA)[NA=(n2xe2x88x92m2)xc2xd, where n is the maximum value of the refractive index of the refractive index distribution type plastic light transmitting device, and m is the minimum value of the refractive index of the refractive index distribution type plastic light transmitting device]. Consequently, there will a problem that Tg lowers substantially, and the heat resistance decreases. On the other hand, if the diffusion material is dispersed to a level close to the limit of the solubility of the diffusion material in the matrix resin in an attempt to increase NA, there will be a problem that micro phase separation tends to occur, and consequently, light scattering tends to increase.
Further, in such a refractive index distribution type plastic light transmitting device, the different refractive index material is merely dispersed in the matrix non-crystalline polymer and is not fixed. Accordingly, there is a problem that when exposed to a high temperature, diffusion or migration takes place, whereby the refractive index distribution changes, and light scattering increases due to coagulation, and consequently the light transmitting performance changes.
On the other hand, as a method of obtaining a light transmitting performance by dispersing a compound having a high refractive index and having good compatibility in a matrix non-crystalline polymer, JP-A-63-106705 proposes a method of forming a refractive index distribution by copolymerizing a monomer comprising a metal salt of a carboxylic acid. But the light transmission loss is at least 800 dB/km (wavelength: 633 nm), and it is impossible to use such a material as a communication medium for a light transmission distance of at least 50 m.
Further, JP-A-5-88026 discloses an optical waveguide having a light amplifying function, or JP-A-7-5505 discloses a light amplifying optical fiber. In each case, the production is a core-clad light transmitting device having a rare earth metal complex dispersed in the matrix polymer of the core portion, but the content of the rare earth metal complex may be at a level of from 1 to 2 wt % for the purpose of light amplification, and the light transmission distance of such a product is at a level of from a few cm to a few m. Further, Appl. Phys. Lett. Vol. 71(17), 2412 (1997) discloses a light amplifying fiber of a refractive index distribution type made of a combination of Eu(TFAA)3(tris-trifluoroacetone europium complex) with a low molecular compound such as triphenylphosphate (TPP), but the content of the rare earth metal complex is at a level of 800 ppm at the maximum, and the light transmission distance is at a level of a few m.
It is an object of the present invention to solve the problems of the conventional refractive index distribution type plastic light transmitting devices and to provide a plastic light transmitting device which is excellent in heat resistance and which has a low light transmission loss and a long light transmission distance.
The present inventors have conducted extensive studies to solve the above problems and as a result, on the basis of an idea that in order to obtain a plastic light transmitting device having improved heat resistance and low light scattering loss, it is important to control Tg of the optical resin material to be low and to reduce the dispersibility of the diffusion material itself, by selecting a compound having a molecular weight larger than a conventional diffusion material known, for example, from WO93-08488, it has been found effective to use, as such a compound, a metal chelate compound comprising an organic ligand and a metal. The present invention has been accomplished on the basis of this discovery. Namely, the present invention provides a refractive index distribution type light transmitting device comprising a non-crystalline polymer (a) and a metal chelate compound (b) having a difference in refractive index of at least 0.001 as compared with the non-crystalline polymer (a) and having a molecular weight of at least 400, wherein the metal chelate compound (b) is distributed in the non-crystalline polymer (a) with a concentration gradient in a specific direction.
Further, the present invention provides such a refractive index distribution type light transmitting device, wherein the metal chelate compound (b) is a metal chelate compound containing a fluorine-containing compound as a ligand.
Further, the present invention provides such a refractive index distribution type light transmitting device, wherein a metal in the metal chelate compound (b) is a rare earth metal.
Further, the present invention provides such a refractive index distribution type light transmitting device, wherein the maximum content of the metal chelate compound (b) in the non-crystalline polymer (a) is from 1 to 20 mass %.
Further, the present invention provides such a refractive index distribution type light transmitting device, wherein the non-crystalline polymer (a) is a non-crystalline fluorine-containing polymer having a cyclic structure in its main chain and having substantially no Cxe2x80x94H bond.
Further, the present invention provides such a refractive index distribution type light transmitting device, wherein the light transmitting device is a light transmitting fiber.